The present invention relates to a device for dewatering material suspensions. The device comprises two co-operating cylindrical rotary rolls, where at least one of the rolls is liquid permeable. The rolls form a nip between them, through which the material is intended to pass downward from above while simultaneously being dewatered.
The material suspension can be, for example, a pulp suspension, and the dewatering device can then be a washing press or a dewatering press.
In the following description a dewatering device according to known art will be described, where both rolls are liquid permeable. The rolls are formed with liquid permeable shell surfaces, which consist of a perforated metal sheet attached to a roll body. In order to gain high capacity, the total open hole area should be great, and at the same time the holes must be small to prevent fine material, for example small fibers, from following along with the liquid through the holes. For solving these problems, the rolls can be provided with a wire cloth on the outside of the perforated shell sheet. The wire cloth allows liquid flow also along the shell surface, and is usually formed of plastic wires. This implies that the perforations can be given a greater size in order to yield a higher capacity without the risk of an increase in the passage of fine material. The wire cloth is used above all when the pulp suspension to be treated contains much fine material, which is the case, for example, with mechanically produced pulps.
The rolls are mounted so as to be rotary on two parallel shafts. The material suspension is supplied to the device, so that it is moved by the rotation of the rolls downward from above through the nip, whereby compression and dewatering of the material to a desired dry matter content takes place.
After the nip, the pick-up of the material from the rolls takes place with the help of a doctor means and the force of gravity. The material drops down into a transport means and is conveyed away from the dewatering device.
Against the shell surface of each roll a doctor blade abuts, which extends along the entire length of the roll. The roll can have a length of up to about 8 m. The object of the doctor blade is to ensure the pick-up of the material from the rolls and to scrape it off mechanically. In the case of rolls with wire cloth, the doctor blades must be arranged at a small distance, usually 0.1 to 0.5 mm, from the roll in order to prevent the doctor blades from cutting the wire cloth to pieces.
A dewatering device of this kind is described, for example, in Swedish Patent No. 504,011.
In the case when only one of the rolls is liquid permeable, it is formed as described above and consequently has a doctor means for picking up the material. The liquid impermeable roll in this case is provided substantially to make the formation of a nip possible.
In a dewatering device of this kind problems arise with the re-wetting of the dewatered material. When the dewatered material leaves the nip, the compressing force decreases. Pressed-off liquid (filtrate) can then run back through the perforated sheet and re-wet the material, which deteriorates the washing result and dry matter content.
In a dewatering device of this kind problems also arise due to the wear of the doctor blades and deflection of same. The doctor means is subject to stringent requirements, whereby the doctor blades should abut the roll, but the force between the blade and the roll must not be so great that the blade and roll are subjected to great mechanical stress. In spite of accurately designed and rigid doctor means, problems arise with wear of the doctor blades and rolls. The doctor blades must be exchanged often because of wear. They are expensive with regard to both their design and manufacture. Wear of the rolls also causes deterioration of their operation and shortening of their life.
When a roll is covered with a wire, an interspace of tenths of a millimeter must be maintained between the doctor blade and the wire. One wants the doctor blade to be located as close to the wire as possible, but without cutting the wire to pieces. This is not made easier by the fact that the doctor blades get worn, and the wire is thus cut to pieces by the doctor blades.
The present invention offers a solution to the aforesaid problems. The functioning of the invention presupposes that the material suspension to be dewatered after the nip has such a dry matter content, that it gets free from the roll, which is facilitated if the dewatered material forms a coherent material web after the nip.
Experiments have shown that the filtrate, which runs back after the nip, preferably places itself like a film of liquid on the roll. This liquid film then follows along with the roll during its rotation. The doctor blade, which according to known art abuts the roll, scrapes off this film, so that it follows along with the dewatered material and re-wets the same. The liquid runs along the doctor blade and down into the transport meets where it re-wets the material.
In accordance with the present invention, these and other objects have now been realized by the invention of apparatus for pressing liquid from a liquid-containing material comprising a pair of juxtaposed rotary rolls forming a nip therebetween, at least one of the pair of juxtaposed rotary rolls being liquid permeable, whereby the liquid-containing material can pass downwardly toward the nip between the pair of juxtaposed rotary rolls for pressing the liquid from the liquid-containing material thereby providing a pressed-out liquid stream and a liquid reduced material, the at least one of the pair of juxtaposed rotary rolls including an abduction surface extending laterally across the at least one of the pair of juxtaposed rotary rolls whereby at least a portion of the pressed out liquid stream runs along the abduction surface, liquid collection means for collecting the portion of the pressed-out liquid stream extending laterally across the at least one of the pair of juxtaposed rotary rolls, the liquid collection means including a liquid inlet for the pressed-out liquid stream, and return flow protection means for preventing the pressed-out liquid stream from rewetting the reduced liquid material. In a preferred embodiment, the abduction surface includes an upper end portion including at least one doctor blade for scraping the pressed-out liquid stream from the surface of the at least one rotary roll between the nip and a predetermined location on the roll where the pressed-out liquid stream is released from the roll.
In accordance with one embodiment of the apparatus of the present invention, the abduction surface includes an upper end portion comprising at least one screen member including an upper end adjacent to the roll and displaced a predetermined distance from the roll whereby a film of the pressed-out liquid disposed on the at least one of the pair of juxtaposed rotary rolls below the nip can pass between the upper end of the screen member and the at least one of the pair of juxtaposed rotary rolls before being released from the at least one of the pair of juxtaposed rotary rolls and the reduced liquid material passes on the side of the screen member distal from the at least one of the pair of juxtaposed rotary rolls. In a preferred embodiment, the predetermined distance is at least about 2 mm.
In accordance with one embodiment of the apparatus of the present invention, both of the pair of juxtaposed rotary rolls are liquid permeable.
In accordance with another embodiment of the apparatus of the present invention, the liquid inlet comprises a gap formed laterally along the entire length of the at least one of the pair of juxtaposed rotary rolls.
In accordance with another embodiment of the apparatus of the present invention, the liquid inlet comprises a plurality of substantially horizontal longitudinally extending rods forming a corresponding plurality of narrow gaps therebetween.
In accordance with the present invention, and in order to prevent the liquid running along the doctor blade from re-wetting the material, a liquid abduction means, or a gill, is provided. The liquid running along the doctor blade is thus led into the gill instead of being allowed to run down into the transport means. The re-wetting is thereby reduced considerably.
When great amounts of liquid are scraped off the rolls, it may happen that part of the liquid is released from the doctor blade before the liquid has been led into the gill. According to the present invention, the doctor blade can then be replaced by a screen means. The screen means is arranged so that the distance between the screen means and the roll is so great, that the filtrate liquid film on the roll passes between the screen means and the roll and thereafter, without coming into contact with the dewatered material, runs down into filtrate collection means. The function of the gill in this case is to catch liquid which, for example at an occasionally thicker filtrate liquid, film, does not pass between the screen means and the roll, but runs along the screen means on its side facing the material web. Due to the fact that the doctor blade in this case is removed, the mechanical effect between the roll and the doctor blade is eliminated, and thereby also all costs in connection with exchange of worn doctor blades are eliminated. There is also no risk that the wire cloth or roll can be damaged.
The requirement on accuracy during the forming of the screen means and on its rigidity are much lower than with a doctor means, because the screen means is located at a relatively great distance from the roll. The present invention, therefore, results in a reduction of costs related to design and manufacture compared to the use of doctor means.
With a device according to the present invention the dry matter content of pulp can be increased from about 2.5 to 8%, to up to about 20 to 40%, preferably from about 3 to 8%, up to about 30 to 40%.